Hitherto, an adhesive sheet comprising a pressure-sensitive adhesive layer which contains a foaming agent, having formed thereon a pressure-sensitive adhesive layer which does not contain a foaming agent is known as disclosed in, for example, JP-A-63-186791 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). This is to overcome the difficulty that a conventional adhesive sheet prepared by simply forming a foaming agent-containing pressure-sensitive adhesive layer on a substrate is difficult to show a sufficient adhesive strength to a material to which the adhesive sheet is adhered due to the decreased adhesive force caused by containing the foaming agent [see, JP-B-50-13878, JP-B-51-24534 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-A-56-61468, JP-A-56-61469, JP-A-60-252681, etc.).
However, formation of the pressure-sensitive adhesive layer which does not contain a foaming agent can achieve the increase of the adhesive strength to a material to be adhered, but there is a problem that when the foaming agent is foamed or the adhesive sheet is subjected to an expanding treatment by heating, it sometimes occurs that decrease of the adhesive strength by the reduction of the adhered area is insufficient and a satisfactory peeling effect does not exhibit.